ON September 1 occurs the tercentenary of the death of the French ecclesiast, mathematician and philosopher, Marin Mersenne, whose correspondence with his contemporaries, published in 1933–37, sheds much light on the history of physical science. Born on September 8, 1588, at La Souitiéde, Sarthe, he was educated at the Jesuit College at La Flêche, where for a short time he had Descartes—eight years his junior—as a school-fellow. From La Flêche, Mersenne went to Paris to study at the Sorbonne, and about 1616 entered the religious order of Minim Friars, an offshoot of the Franciscans. From 1616 until 1619 he was professor at the College of Nevers and thence onwards was superior of the Convent of his Order in Paris. He travelled into Germany, Holland and Italy, translated Galileo‘s "Mechanics", and held small gatherings of mathematicians and the like in his room in the Convent, thus anticipating the inauguration of the Royal Academy of Sciences.